ST92F124 datasheet(258/429 Pages) STMICROELECTRONICS

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SERIAL PERIPHERAL INTERFACE (SPI)

SERIAL PERIPHERAL INTERFACE (Cont’d)

10.7.4.7 Single Master and Multimaster Configurations

There are two types of SPI systems:

– Single Master System

– Multimaster System

Single Master System

A typical single master system may be configured,

using an MCU as the master and four MCUs as

slaves (see Figure 125).

The master device selects the individual slave de-

vices by using four pins of a parallel port to control

the four SS pins of the slave devices.

The SS pins are pulled high during reset since the

master device ports will be forced to be inputs at

that time, thus disabling the slave devices.

Note: To prevent a bus conflict on the MISO line

the master allows only one slave device during a

transmission.

For more security, the slave device may respond

to the master with the received data byte. Then the

master will receive the previous byte back from the

slave device if all MISO and MOSI pins are con-

nected and the slave has not written its SPDR reg-

ister.

Other transmission security methods can use

ports for handshake lines or data bytes with com-

mand fields.

Multi-Master System

A multi-master system may also be configured by

the user. Transfer of master control could be im-

plemented using a handshake method through the

I/O ports or by an exchange of code messages

through the serial peripheral interface system.

The multi-master system is principally handled by

the MSTR bit in the SPCR register and the MODF

bit in the SPSR register.

Figure 125. Single Master Configuration

MISO

MOSI

MISO

SCK

Slave

MCU

Slave

MCU

Slave

MCU

Slave

MCU

Master

MCU

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